Recently, with the developments of a semiconductor production industry, an optoelectronics production industry, a precision instrument production industry, a hard metal material production industry, an ornaments production industry, etc., various gases have been used.
Of those gases, harmful gases such as monosilane, disilane, trimethylsilane, triethylsilane, trimethyl gallium, triethyl gallium, trimethyl indium, triethyl indium, etc., in addition to ammonia are used in a nitride film production step for the production of compound semiconductors. These gases are indispensable materials in the production steps of compound semiconductors, but these gases each has a high toxicity and when unreacted gases are released in the air after use, they give bad influences on the human bodies and the environment.
The allowable concentrations of these gases are, for example, that the concentration of ammonia is 25 ppm, and the concentration of monosilane is 5 ppm. Therefore, the gases containing these harmful gases must be cleaned prior to releasing in the air after use in a semiconductor production step, etc.
Of these harmful gases, in particular, an ammonia gas is sometimes used as a mixture with a hydrogen gas or a nitrogen gas at a relatively high concentration of the ammonia gas of from 1 to 30 vol %. In addition, monosilane, disilane, trimethylsilane, triethylsilane, and the like are further added thereto at a concentration of from 1 to 1,000 ppm, and trimethyl gallium, triethyl gallium, trimethyl indium, triethyl indium, and the like are further added thereto at a concentration of from about 0.01 to 1 vol %.
The gas mixture containing these harmful gases is used in a compound semiconductor production step, and the unreacted gas is then made harmless by removing the harmful gases prior to release in the air. However, a method of efficiently cleaning the gas mixture at a low cost has not yet been developed and thus the development of such a method has been strongly desired.
Conventional cleaning methods for an exhaust gas containing ammonia in a relatively high concentration and also containing a silane such as monosilane, disilane, or trimethylsilane, and an organometallic compound such as trimethyl indium, or trimethyl gallium are (1) a method of contacting the exhaust gas with an aqueous solution of sulfuric acid, etc., to absorb the exhaust gas into the aqueous solution, thereby catching ammonia as ammonium sulfate and other harmful components as the oxides or sulfates in the aqueous solution; (2) a method of introducing the exhaust gas into a combustion furnace and removing ammonia as water and nitrogen, and other harmful components as water, a carbonic acid gas, and oxides; (3) a method of contacting the exhaust gas with each dry-type cleaning agent corresponding to each harmful component contained in the exhaust gas; and the like.
However, the above methods have the following disadvantages.
That is, in the method (1) of contacting the exhaust gas with the aqueous solution of sulfuric acid, etc., and absorbing the harmful components in the solution, the harmful gases such as ammonia, monosilane, trimethyl gallium, trimethyl indium, etc., can be removed, but there are inconveniences that the removal ratio thereof is low, whereby the concentrations of the harmful gases cannot be lowered to the environmental standard allowable concentrations, and also that a large amount of ammonium sulfate is by-produced in the cleaning treatment, and solid matters such as silicon oxides, indium oxides or gallium oxides are contained in the treating solution. In the method (2) of making the exhaust gas harmless by introducing the exhaust gas into a combustion furnace and burning the harmful components in the furnace, not only there is an inconvenience that since the exhaust gas is not released under a stationary state, the method involves a danger of an incomplete combustion, an abnormal combustion, or an explosion but also there is a fault that nitrogen oxides which are harmful materials are newly by-produced by the combustion treatment. In the case of the method (3) of making the exhaust gas harmless by contacting the exhaust gas with a dry-type cleaning agent, by using the combination of each cleaning agent according to the kind of a harmful gas, all the harmful components can be removed but there are faults that a large amount of a cleaning agent is required for cleaning an ammonia gas contained in the exhaust gas in a relatively high concentration and a very large treatment apparatus is required for the method.
As described above, the development of an exhaust gas cleaning method, wherein the cleaning apparatus is small-sized, the cleaning efficiency is high, and the treatment after cleaning can be easily carried out, has been strongly desired.